Use Decimate modifier as a realtime LOD generator
To add a driver to the Decimate Modifier's Ration value, right click on it and choose 'Add Driver'. The slider will turn purple.



Go to the Graph Editor and choose the Drivers context.



For the driver Type choose Distance.

For one object choose your Camera, and for the other your object with the modifier on it. Name your variable something that makes sense to you (mine is 'dist'). For the scripted expression you can use 1/(dist/8) for example and the Ratio value will decrease as the distance increases. 

You can replace 8 with a lower number for less distance required for decimation, or a higher number for more distance required.



As for setting a minimum of 0.2, I don't know how you could do that, but I bet if you look in the Python documentation or maybe ask on Stack Overflow you can find out how to write that specific expression.